In recent years, the displays provided in TV sets and mobile apparatuses have shown remarkable progress in resolution level. Through the last decade, in which the high-vision broadcasting started and the Blu-ray Disc appeared on the market, the resolution of the display has evolved from SD (for example, 720×480) to HD (for example, 1366×768), and then to Full HD (for example, 1920×1080).
However, the progress in resolution of the display naturally leads to an increase in amount of image data to be outputted to the display. In addition, the image processing unit that outputs the image to the display is required to carry a frame memory having a larger capacity for temporarily storing the outputted data, and to utilize a wider memory bandwidth. With the increase in data amount, such issues as an increase in number of external memories to be connected and in power consumption have become more critical.
Here, a popular digital television (DTV) system includes a decoder block and an image signal processing block.
The decoder block decodes stream data based on a standard such as MPEG2 or H.264 and writes the image (frame) data in a memory.
The image signal processing block reads the image data written in the memory and performs image processing such as resizing and upgrading of image quality. The image signal processing block also synthesizes an on-screen display (OSD) and a plurality of motion pictures. The image signal processing block temporarily writes the image data to be subjected to the image processing or synthesizing in the memory, and reads out and outputs the processed image data when such data is required.
More specifically, in the case where a 24-bit full-color picture having a resolution of 1080 p (1920×1080, 60 fps) is to be processed, the necessary bandwidth per frame is approximately 360 MB/s. Further, in the case where the picture is subjected to a complicated image processing, such as synthesis of a plurality of motion pictures, a considerably higher memory bandwidth is required because of the significant increase in number of writing times and reading times in and out of the memory. In such a case, the DTV system has to have a plurality of high-frequency memories having a bandwidth of several GB/s or higher, which naturally leads to an increase in cost.
Accordingly, compressing the image data before storing the image data in the memory to thereby reduce the amount of the image data to be stored therein allows the memory bandwidth to be reduced and, for example, allows the number of memories to be installed to be reduced and a memory of a lower frequency to be employed, which contributes to reducing the cost.
Some conventional image compression devices are configured to divide image data into a plurality of image blocks and compress each of the image blocks. In this case, color difference components contained in the data set value of the image block are subsampled (decimated) depending on whether the image block contains a CG edge, and the data set value obtained through the subsampling is encoded by a predictive coding method (for example, see PTL 1). FIG. 11 is a block diagram showing a configuration of the image compression device disclosed in PTL 1.
The image compression device according to PTL 1 converts the color space of the inputted image data into luminance components and color difference components, and modifies (reduces) the resolution with respect to a part of the plurality of components, in accordance with gradation increments in the inputted image data. The image compression device according to PTL 1 is configured to reduce the resolution of the components that are not largely affected by the reduction in resolution, such as Cb components and Cr components of image data having YCbCr color space, and B components of image data having an RGB color space.